When an ion arc is struck between the anode and the cathode, the material is locally atomized under the influence of the heating. The ionized gas causes a plasma which is formed by a mixture of ions and electrons having a total charge which is equal to zero. The ion arc is sometimes initiated by an auxiliary plasma between the anode and the cathode by means of an independent control electrode for a period of time which is short with respect to the length of the arc pulse.
The formation of the plasma with an average energy of a few tens of electron volts occurs from very bright spots of very small dimensions, which are termed cathode spots, and the plasma is in the form of a cone, the conical angle of which is approximately 30.degree..
For certain materials the plasma formation is associated with an emission of micro drops of molten material; and the emission is not isotropic and for the greater part is located in a solid angle near the surface of the cathode.